What does <//> mean in Haskell? - haskell

Question

From the following code from a simple server using Spock:
{-# LANGUAGE OverloadedStrings #-}
module Main where
import Web.Spock
import Web.Spock.Config
import Data.Time.Clock
import Control.Concurrent
import Network.HTTP.Types.Status
import Network.HTTP.Types.URI
import Control.Monad.Trans
import Control.Concurrent.STM
import qualified Data.Text as T
app :: MyApp ()
app =
do get root $ redirect' "https://google.no"
-- Store params
get ("oauth2" <//> var) $ \path' ->
do ...
The majority of the imports don't relate to the question.

Answer 1

This is the (<//>) :: Path as Open -> Path bs ps -> Path (Append as bs) ps function. As the documentation says:
Combine two path components
In the source code [GitHub], we see that it is implemented as:
(<//>) :: Path as 'Open -> Path bs ps -> Path (Append as bs) ps
(<//>) = (</>)
This (</>) function originates from the (</>) :: Path as Open -> Path bs ps -> Path (Append as bs) ps in the reroute package. It is implemented as [GitHub]:
(</>) :: Path as 'Open -> Path bs ps2 -> Path (Append as bs) ps2
(</>) Empty xs = xs
(</>) (StaticCons pathPiece xs) ys = StaticCons pathPiece (xs </> ys)
(</>) (VarCons xs) ys = VarCons (xs </> ys)
It thus basically appends some path pieces together. You can here see this as some sort of linked list. A string literal (like "oauth2") can, with the OverloadedStrings be converted into a Path since it is an instance of the IsString class [GitHub]:
instance (a ~ '[], pathState ~ 'Open) => IsString (Path a pathState) where
fromString = static
It will generate a StaticCons with each time a piece of the path (well since "oauth2" does not contain any slashes, it will just have one block):
static :: String -> Path '[] 'Open
static s =
let pieces = filter (not . T.null) $ T.splitOn "/" $ T.pack s
in foldr StaticCons Empty pieces